"Silence! SILENCE!...Anyone who has been splashed, come here for a Deflating Draft - when I find out who did this -"
-- Snape after the Swelling Solution in Goyle's cauldron exploded (CS11)
Antidote to Swelling Solution.
References from the canon
Snape had some on hand while Harry's second year class was preparing Swelling Solution, which turned out to be a handy thing (CS11).